Usually, a hearth roll disposed within continuous heat treatment furnaces continuously anneals and conveys steel sheets having not been subjected to heat treatments for extended time periods under a reduced atmosphere at 500 to 1000° C. Accordingly, the roll surface is abraded, and oxides and iron powder attached to the steel sheets are adhered and deposited on the roll surface. Thus, so-called build-up is often formed.
When unevenness resulting from such abrasion and build-up is generated on the surface of the hearth roll, steel sheets are scratched while being conveyed, causing reduction in quality. For preventing the reduction in quality of steel sheets attributable to the hearth roll, the operation of heat treatment furnaces is periodically interrupted for performing maintenance works such as polishing the hearth roll surface and replacing the roll.
Patent Literature 1 discloses a method including: thermally spraying ceramics or cermets on a metal or alloy coating film formed on the surface of a hearth roll; impregnating the top of this thermal spray layer with an aqueous solution containing chromic acid (H2CrO4) as a main component to seal the coating film; and subsequently performing a burning treatment to form oxides of various metals and a Cr2O3 ceramics coating film (see paragraph 0014 in the specification.)
Patent Literature 2 discloses a chromate treatment including: soaking, coating, or spraying a thermal spray coating film formed on the outermost layer of a hearth roll with an aqueous solution containing chromic acid (H2CrO4); and thereafter performing burning at 350° C. to 550° C. to form a film (see paragraphs 0018, 0021 and the like in the specification). Patent Literature 3 also discloses a chromate treatment which is substantially similar to Patent Literature 2.